The present invention relates to a combustible composition in the form of an emulsion, to a process for its production from heterogeneous fatty waste and to a combustion process using this combustible composition.
Organic waste such as that from the agro-food industry (meat flour, viscera, etc.) are characterized by a moderate gross calorific value but cause enormous problems with combustion, in particular because of their substantial heterogeneity.
Thus when supplying combustion apparatus with such waste, the heterogeneity of the waste renders it difficult to manipulate and combustion regulation is hazardous. Combustion furnaces supplied by that type of waste are subjected to substantial and rapid variations in calorific value: such a modus operandum is technically unacceptable for a conventional combustion facility.
Further, because of the heterogeneity of the waste, the combustion furnace can come into contact with either a fraction of waste with a very high volatile matter content or with a waste fraction which is depleted in volatile material. Whatever the case, combustion causes problems: in the first instance, combustion is very rich and the temperatures reached in the furnace cause material resistance problems; in the second case, combustion is poor and results in residues and unburned material.
The present invention aims to overcome the problems mentioned above.
In particular it aims to provide, from heterogeneous fatty organic waste, a stable homogeneous combustible composition in the form of an emulsion. It also aims to provide a process that can easily produce such a combustible composition. It further aims at providing a combustion process which has none of the disadvantages observed during combustion of untreated fatty waste.
In general, the combustible composition of the invention consists of a water-in-oil and/or organic compound emulsion, which is homogeneous and stable, formed from heterogeneous fatty waste and further containing solid particles in suspension.
In the combustible composition of the invention, the water is in the form of droplets dispersed in the continuous phase, which is an organic phase or a fatty phase.
It should be noted that, in terms of combustion, an emulsion of water in an organic material is more interesting than an emulsion of an organic material in water, as if the continuous phase were water, sufficient energy would have to be added to the combustion apparatus to evaporate the water surrounding the organic material before triggering combustion, while if the continuous phase were the organic material, it would be sufficient to supply energy to reach the ignition temperature of the fatty phase to trigger combustion: in the solution recommended by the invention, the heat released on combustion is used to evaporate the water contained in the composition.
Depending on the nature of the fatty waste treated to prepare the combustible composition of the invention, the distribution by weight between the different constituents can be as follows:
10% to 60% by weight of water;
10% to 60% by weight of fats; and
1% to 80% by weight of solid matter.
A preferred weight distribution is:
10% to 55% by weight of water;
15% to 45% by weight of fats; and
3% to 75% by weight of solid matter.
The solid matter generally contained in the combustible compositions of the invention are generally in the form of pieces with relatively homogeneous sizes with their largest dimension of 10 cm or less, preferably 2 cm or less and more preferably, 1 cm or less.
The combustible compositions of the invention generally have a net calorific value (NCV) of at least 6000 kJ/kg, preferably at least 10000 kJ per kilogram.
In general, production of the combustible compositions of the invention comprises two principal operations:
a) reducing the granulometric heterogeneity of the waste; and
b) forming an emulsion by agitation, and optionally adding an emulsifying agent.
The two operations can be combined into a single step or dissociated into a plurality of steps carried out in succession.
More particularly, the starting fatty waste can consist of all sorts of waste from the agro-food industry, animal flour, waterfowl viscera, etc.
By way of indication, the proportion of fats in animal flour is generally 10% to 35% by weight. This proportion can, for example, be 20% to 40% by weight in viscera. In these, the proportion of water can, for example, be 35% to 55% by weight.
To reduce the heterogeneity of fatty waste, it is subjected to a homogenisation operation (a) termed a xe2x80x9cdynamic homogenisation operationxe2x80x9d which can, for example, consist of shredding, grinding, chopping or any other operation enabling the size difference between the various solid components of the waste to be reduced. This operation can be carried out, for example, in a grinder, a chopper or any other apparatus that can shear the fatty waste pieces. The operation can be carried out in batches or continuously.
As already indicated above, the granulometry which is to be obtained can correspond to particles with a variety of shapes, the largest dimensions of which can be 10 cm or less, preferably 2 cm or less, and more preferably, 1 cm or less.
The temperature at which homogenisation is carried out is generally less than 70xc2x0 C. and preferably less than 55xc2x0 C., for reasons which will be given below.
Emulsion formation or structure homogenisation (operation (b) in the production process of the invention) is carried out by agitating the fatty waste after or simultaneously with operation (a) carried out above.
The water in the emulsion composition constituting the dispersed phase can originate from the treated organic waste. It can also be water added during the emulsion formation step. In general, it is preferable not to add water.
The emulsion is formed by agitation and optional addition of an emulsifying agent which is preferably soluble in the organic (or fatty) phase. Compounds with a low hydrophilic/lipophilic balance (HLB) are selected. This balance, which represents the relative weight of the fatty chain of the molecule to the solubilising group, is in the range 2 to 8, preferably in the range 3 to 6.
Groups of emulsifying compounds encouraging the formation of an emulsion of water in the fat include ethoxylated or propoxylated alcohols, fatty acids or esters, amines or amides with high molecular weights. Preferably, higher fatty alcohols are selected, such as sterols or polyglyol mono- and di-esters with a short polyglycolic chain.
The amount of such emulsifying compounds with respect to the overall weight of the liquid mixture, fat plus water, will be in the range 0.5% to 5% by weight and preferably 1% to 3% by weight.
Sulfonated or sulfated oil type emulsifying agents can also be used, but during combustion they liberate non-negligible quantities of oxides of sulfur.
Pairs of emulsifying agents, with different behaviours when taken separately can, when they are associated in suitable proportions, lead to stable water-in-oil emulsions. Examples that can be cited are higher alcohol esters mixed with quaternary ammonium salts.
In a preferred version of the invention, the emulsifying agent is produced in situ by adding to the waste traces of a base generally consisting of an alkali or alkaline-earth metal hydroxide such as sodium, potassium, calcium or magnesium hydroxide, after or during granulometric homogenisation.
Fatty waste contains traces of free fatty acids and also undergoes partial hydrolysis, which liberates fatty acids and alcohols (in general glycerol). As a result, adding an alkaline or alkaline-earth base causes neutralisation of the acids and the formation of alkaline or alkaline-earth soaps, which act as emulsifying agents during emulsion preparation.
If the organic fraction contained in the waste is represented as being in the form of triglycerides, the reactions can be schematically represented as follows: 
The hydrolysis reaction is encouraged by raising the temperature, so it is recommended that the dynamic homogenisation reaction operation (a), intended to reduce the granulometric heterogeneity, be carried out at low, controlled temperatures, for example less than 55xc2x0 C., as advanced hydrolysis causes an increase in the acidity of the waste, which necessitates the use of larger amounts of base to neutralise the acids. This phenomenon is a disadvantage for two reasons; the first is economic and the second is linked to combustion. In fact, the addition of large quantities of bases reduces the calorific value of the final fuel and causes combustion residues in the form of alkaline salts.
As a result, in a preferred version of this invention, the dynamic homogenisation operation (a) is carried out at temperatures of less than 70xc2x0 C., preferably less than 55xc2x0 C. for the reasons given above, and to prevent any breakdown phenomenon causing rupture of the emulsion.
The emulsification operation (b), the choice of the neutralising base or the emulsifying agent is fundamental. The present invention seeks to produce a fuel in the form of an emulsion of water in the organic matter and not of organic matter in water, preferably simply by using the water initially contained in the waste.
In the case of using a neutralising base, a concentrated aqueous solution with a content of 10% to 40% by weight, preferably 20% to 30% by weight is preferably used. Too dilute a base can in effect cause the formation of an emulsion of organic matter in water, and a reduction in the calorific value of the fuel.
In order to produce a fuel in the form of a water-in-organic matter emulsion, it is vital that the alkaline or alkaline-earth soap formed by adding a base to the waste, or the emulsifying agent, is more soluble in the organic fraction composing the waste than in the water.
Production of the fuel necessitates, in operation (b), intimate contact between the waste and the emulsifying agent or the neutralising base. It is recommended the waste-emulsifying agent mixture and/or neutralising base be agitated for 3 to 4 minutes using an apparatus which can provide a mixing energy in the range 50 to 500 kJ per kilogram of waste, preferably in the range 100 to 250 kJ per kilogram of waste.
Steps (a) and (b) for producing the fuel as mentioned above are given by way of indication. This does not exclude the production of the fuel in a single step combining granulometric homogenisation and emulsion production.
In accordance with the invention, it is possible to add an agent which blocks fermentation to the organic phase of the emulsion and, in order to increase the calorific value of the combustible composition, one or more hydrocarbon fuels such as a fuel oil.
The combustible compositions of the invention obtained from heterogeneous fatty waste are in the form of homogeneous and stable emulsions, which are easy to manipulate and to use in a variety of types of conventional combustible facilities such as horizontal furnaces, vertical furnaces, step furnaces or step-heart furnaces, rotary furnaces, fluidised bed- or circulating bed furnaces or rotary cupola furnaces or any other furnace type which can admit liquid and/or paste products.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding French application 98/12751, filed Oct. 12, 1998, are hereby incorporated by reference.